FIELD OF THE INVENTION
The invention relates to a method for distinguishing causes of error in the mixture forming or mixture regulating system of an internal combustion engine, in which a fuel-air mixture supplied to the engine is regulated to a set-point or command value on the basis of an output signal of a heated lambda sensor, with the aid of a lambda regulator and the lambda sensor disposed in an exhaust system of the engine.
In order to keep the proportions of pollutants in the exhaust gas in an internal combustion engine low, it is important to keep the air-fuel ratio of the mixture supplied to the engine at an optimal, previously set value. In order to do so, regulating devices are used that work as a function of a signal furnished by an exhaust gas sensor which is disposed in the engine exhaust system and is known as a lambda sensor. That signal is compared with a reference voltage corresponding to an optimal value, and a control signal for varying the fuel-air delivery is derived from the comparison.
The prerequisite for proper functioning of such a regulating device is that the lambda sensor function perfectly. In the known lambda sensors, which ascertain the oxygen concentration in the exhaust gas, functional readiness is not assured until a certain temperature is reached. In order for the lambda sensor to reach its operating temperature as fast as possible, and so that the sensor temperature can thereafter be kept at a predetermined constant value, an additional heater is provided, which not only assures that the lambda sensor will be heated by the exhaust gases themselves but also assures rapid operational readiness.
The lambda sensors used in such devices are constructed in such a way that at a rich air-fuel mixture, they output a relatively high voltage, and at a lean air-fuel mixture they output a low voltage as compared with a rich mixture composition. The transition from the high to the low voltage is virtually abrupt at the air number lambda =1, because at air numbers that are slightly greater, uncombusted oxygen is suddenly present in the exhaust gas.
On one hand, if the mixture is lean (lambda &gt;1), the voltage output by the lambda sensor is thus near zero (a few mV, for instance) and cannot be distinguished, or can only be distinguished with difficulty, from a break in the supply wires to the lambda sensor (referred to below as a line break) or from a short circuit of the signal line to ground. On the other hand, however, since the output voltage of the lambda sensor is relatively high with a rich mixture (lambda &lt;1), and since even in a short circuit of the lambda sensor line toward the on-board electrical voltage or toward the supply voltage of the electronic control unit the output voltage can assume values that are above a limit value for the rich mixture and therefore can incorrectly indicate that a rich mixture is present, it is again necessary to find out what type of error is involved.